1. Field of the Invention
The present invention is generally concerned with the trimming operation whereby an eyeglass lens is machined to match the contour of the rim or surround of the eyeglass frame in which it is to be mounted, independently of the finishing operation which usually follows on from the trimming operation and in which a bevel or a groove can be formed on the edge of the eyeglass frame, depending on how it is attached to the rim or surround of the frame.
2. Description of the Prior Art
The trimming operation is usually carried out on a trimming machine in which the cutting tools are grinding wheels or milling tools, for example; this machine is usually called a grinder or bevelling grinder.
The present invention is more particularly directed to the situation in which a trimming machine of this kind is associated with a contour follower device, more particularly still with the situation in which the system comprising the trimming machine and the contour follower device is under the control of a numerical controller.
As the corresponding arrangements are well known in themselves and do not of themselves constitute any part of the present invention they need not be described here.
Suffice to say that the contour follower device can read off the shape in terms of its magnitude and direction, i.e. orientation, either directly from the rim or surround of the eyeglass frame or indirectly from a template imaging the rim or surround of the eyeglass frame.
In either case the numerical controller then supplies the trimming machine with the data obtained in this way, in order for the trimming machine to reproduce it.
Naturally, after machining there must be as close as possible a fit between the contour of the eyeglass lens and that of the rim or surround of the eyeglass frame in which it is to be mounted.
The results of this machining are subject to two, successive types of error.
There is firstly the reading error due to the contour follower device.
Then there is the reproduction error due to the trimming machine.
The reading error of the contour follower device is for the most part due to its design and in practice is relatively constant.
However, the reproduction error of the trimming machine is essentially due to inevitable wearing down of the cutting tool, and so increases with time.
In both cases, however, these are systematic errors which are always operative in the same direction, rather than randomly, and it is therefore possible to allow for them by means of calibration operations.
Accordingly, at present the contour follower device is usually calibrated before use using a gauge whose known shape is compared with the measured shape. Likewise, at present the trimming machine is usually calibrated in its turn, also using a gauge which forms a template for machining a lens and whose known shape is compared with that of the machined lens.
In the case of the trimming machine, at present this calibration operation entails measuring the machined lens with callipers and must be systematically carried out after each sharpening of the cutting tool. Each time the operator must adjust accordingly the reproduction error to be allowed for in each machining operation.
A first object of the present invention is to provide a method which simplifies determination of the reproduction error.